I'm not sure there's a whole lot of difference between them as far as crash safety is concerned. Perhaps the thin plastic shell is a bit stronger on the higher priced helmet, but this really isn't nearly as important as the density of the polystyrene bead foam that makes the bulk of said headgear. Then, I'd say, comes the retention system.
It is the foam that absorbs the energy from a crash, not the plastic cover. And the foam does this by crushing/displacing. Very like "crumple zones" in cars. In the event of a crash, they're both equally junk. That is, they still both need to be replaced. They do both have to meet minimum standards as far as energy absorbtion goes, but this does not mean that they are not allowed to exceed such standards. Perhaps the spendier helmet can absorb more energy than the lesser priced one. Perhaps, it doesn't. Insofar as I can tell, there isn't any place one can look to determine how much energy any given helmet can absorb or how well it did in various crash tests or how well one fared against another.
I'd get the one that feels better on your head and has the largest vents. The best helmet won't be worn if it is uncomfortable and if it doesn't or cannot be made to fit properly, it won't be any benefit. Big vents are a plus if you ride in hot weather. Visors are nice, too.